1. Field of the Invention
The present invention relates to a light source device and a projection display, and more particularly to a light source device which employs a plurality of light emitting diodes for a light source, and a projection display which incorporates the light source device.
2. Description of the Related Art
Conventional projection displays typically comprise a light source using a single high-pressure mercury lamp which irradiates an illumination beam therefrom to three or one two-dimensional image device for modulating an image. The modulated image is enlarged and displayed on a screen using a projection lens. The high-pressure mercury-arc lamp, which serves as the light source, is a white lamp that has light emission characteristics from an ultraviolet region to an infrared region, and a dichroic mirror, a rotary color filter, or the like is used to select wave-length components of R (red), G (green), B (blue) light required for color display from the white light, for irradiating the two-dimensional image device with the selected light components. Known two-dimensional image devices include a transmission-type and a reflection-type liquid crystal panel, a digital mirror device (DMD), and the like. The foregoing conventional projection display employs a relatively large lamp for the light source because it is generally used in combination with a parabolic or an elliptic reflection mirror. In addition, due to large amount of heat generated by the light source itself, a cooling fan must be used to manage the temperature of the lamp and a cooling condition for the two-dimensional image device when the lamp is on. These requirements make it difficult to reduce the size and weight of the overall display.
JP-2001-249400-A discloses techniques for solving the drawbacks of the conventional projection display. The disclosed techniques involve arranging light emitting diodes or laser diode elements in matrix for use as light sources, instead of a high-pressure mercury-arc lamp. Then, each of the light sources is associated with each elemental lens of a pair of lens arrays to construct an optical system which can efficiently irradiate a two-dimensional image device with illumination beams from the individual light emitting elements. The two-dimensional image device is implemented by a transmission-type liquid crystal panel. The employment of light emitting diodes or laser diode elements for the light sources contributes to the accomplishment of a reduction in the size and weight of the overall display as well as a saving in power consumption of the light sources.
Likewise, JP-2002-244211-A and JP-2002-56410-A disclose techniques for solving the drawbacks of the conventional projection display. As illustrated in FIG. 1, the disclosed techniques involve chromatically combining illumination beams from R, G, B light emitting diode boards 801, 802, 803 with cross dichroic prism 807 for use in illumination of single two-dimensional image device 808. The advantage provided by these techniques is the ability to make an illumination system compact even if a relatively large number of light emitting diodes are used.
However, a light source composed of light emitting diodes or laser diodes, as in the aforementioned prior art examples, disadvantageously fails to commercially provide a projection display bright enough for practical use due to a lack of luminance produced by each light emitting element. To solve the lack of luminance, it is desirable to use the largest possible number of light emitting diodes.
Particularly, when a dichroic mirror is used in a projection display as disclosed in JP-2002-244211-A and JP-2000-56410-A, a limitation is imposed to locations available for disposing R, G, B light emitting diodes, giving rise to the following problems.
Generally, light emitting diodes differ in the light emission efficiency from one another depending on luminescent colors emitted thereby. In other words, even those light emitting diodes in the same family which are identical in chip size, shape and the like, differ from one another in the luminance produced thereby. Giving as an example high-luminosity light emitting diodes at a standard luminosity rank made by Nichia Corporation, a red light emitting diode (model number: NSPR346BS) provides 0.305 candelas of luminosity; a blue light emitting diode (model number: NSPB346BS), 0.5 candelas; and green light emitting diode (model number: NSPG346BS), 1.73 candelas. Giving another example, the Luxeon series made by Lumileds Lighting, Inc. in U.S.A. includes a red light emitting diode (model number: LXHL-BD01) which provides 25 lumens; a blue light emitting diode (model number: LXHL-BB01) which provides 5 lumens; and a green light emitting diode (model number: LXHL-BM01) which provides 25 lumens. Such variations in luminance is thought to result from the materials of the light emitting elements, and the like.
FIG. 2 illustrates an outer appearance of cross dichroic prism 901 mentioned above. When using cross dichroic prism 901, as many light emitting diodes as possible are ideally provided in each of R, G, B channels 902, 903, 904. Cross dichroic prism 901 has incident surfaces for the respective channels which have the same dimensions, so that a maximum number of available light emitting diodes is also the same for the respective channels. While a total light amount is maximized by providing the maximum number of light emitting diodes in each channel, the light emitting diodes differ in optical power from one color to another, so that for ensuring a predetermined white balance, the maximum number is actually limited by a light emitting diode of a particular color, and a less number of light emitting diodes can merely be provided in the other channels. Even if the same number of light emitting diodes are provided in the respective channels, all the light emitting diodes cannot be used in such a manner that the maximum optical power can be produced in each of the channels.
Specifically, NTSC (National Television Standards Code) defines the light amount ratio of R, G, B, which make up white at approximately 3:6:1. Accordingly, with the light emitting diodes made by Nichia Corporation, the light emitting diodes of the R, G, B colors should be provided at the ratio of 2.8:1:0.57, i.e., 2.8 of R light emitting diodes and 0.57 of B light emitting diodes for one G light emitting diode. In other words, the R-channel requires light emitting diodes 2.8 times more than those required by the G-channel, while the B-channel requires light emitting diodes 0.56 times more than those required by the G-channel. With the Luxeon series made by Lumileds Lighting, Inc. in U.S.A., the ratio is such that 0.5 of R light emitting diodes and 0.83 of B light emitting diodes are required for one G light emitting diode. For this reason, even if a cross dichroic prism is specially used in favor of a reduction in size, a resulting display disadvantageously suffers from an insufficient light utilization efficiency because it cannot utilize the maximum power from the maximum number of available light emitting diodes provided in each channel.